True RMS converter board

ABSTRACT

A true RMS conversion circuit. A test apparatus includes a transmitter source including AC and DC supplies. The transmitter source can connect to a transmitter. The test apparatus includes an optical receiver to receive optical signals from a transmitter coupled to the transmitter source. The optical receiver includes an amplifier with a differential output. The test apparatus also includes a matching networks connected to a high and low outputs. Also included in the test apparatus are filters connected to the matching networks. The filters are designed to separate AC and DC signals. An RMS conversion circuit is connected to the filters such that the RMS conversion circuit receives AC signals from the filters. The RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals. A data acquisition system is connected to the RMS conversion circuit to receive the function of the RMS value of the AC signals. The data acquisition system is connected to the filters to receive DC signals from the filters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/599,259, titled “True RMS Converter Board” filed Aug. 4, 2004, whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The invention generally relates to fiber-optic test equipment. Morespecifically, the invention relates to test equipment for testingoptical transmitters and receivers used in fiber-optic communications.

2. Description of the Related Art

Fiber-optic networking can be used to communicate in modern high-speednetworks. To transmit data on a fiber-optic network, the data must beconverted from an electronic signal to an optical signal. Thisconversion may be done, for example, by using a transmitter ortransmitting optical subassembly (TOSA). The transmitters and TOSAsoften include light generating devices such as a laser or light emittingdiode (LED). The light generating device is modulated according todigital data to produce a modulated optical signal.

When optical signals are received, those optical signals must generallybe converted to an electronic signal. This is often accomplished using areceiver or a receiver optical subassembly (ROSA). Receivers and ROSAsgenerally include a photo sensitive device such as a photodiodeconnected to a transimpedance amplifier (TIA). When an optical signalimpinges the photo sensitive device, a modulated current is induced inthe photo sensitive device. This current can be converted by the TIA toan electronic signal usable by digital devices on a network.

Manufacturers of ROSAs and TOSAs typically perform various performancetesting on the ROSAs and TOSAs before they are delivered to distributorsand end customers. This performance testing can be used to detectdefects or to sort components into groups of different rated values.

More particularly, testing directed towards the ROSA may include testingthe responsivity of the ROSA to a modulated optical signal, testing theamount of current produced for a given amount of optical signal and soforth. Testing responsivity includes comparing a modulated opticalsignal input into the ROSA to an AC electrical signal produced by theROSA as a result of receiving the AC optical signal.

Testing may be performed on the TOSA to characterize operatingcharacteristics of the TOSA. One test that may be performed includesplotting the amount of optical energy produced by the TOSA as a functionof the amount of current used to drive the TOSA. Another test includesmeasuring the amount of noise produced by the TOSA.

Many of these tests have conventionally been performed using expensivehigh-frequency test equipment. For example, some tests use a highfrequency communications analyzer costing in the tens of thousands ofdollars. Further, many of these test devices are general-purpose testdevices. As such, these devices require excessive amounts of humaninteraction to perform the test result in and increase test times foreach component. When each and every component manufactured is tested,this requires an inordinate amount of manpower and equipment to processtesting of the components quickly.

Additionally, testing is often not repeatable from part to part. This isdue to the changing nature of cables and the like associated withgeneral purpose test equipment.

BRIEF SUMMARY OF THE INVENTION

One embodiment includes a test apparatus for testing transmitter devicessuch as laser diodes and LEDs. The test apparatus includes a transmittersource. The transmitter source includes an AC supply and a DC supply.The transmitter source is able to connect to a transmitter. The testapparatus further includes an optical receiver configured to receiveoptical signals from a transmitter coupled to the transmitter source.The optical receiver includes an amplifier with a differential output.The test apparatus also includes a first matching network connected to ahigh output of the differential output and a second matching networkconnected to a low output of the differential output. Also included inthe test apparatus are a first filter connected to the first matchingnetwork where the first filter and a second filter connected to thesecond matching network. The first and second filters are designed toseparate AC and DC signals. The first and second filters may be forexample bias tees. An RMS conversion circuit is connected to the firstand second filters such that the RMS conversion circuit receives ACsignals from the first and second filters. The RMS conversion circuitconverts the AC signals to a DC function of the RMS value of the ACsignals. A data acquisition system is connected to the RMS conversioncircuit to receive the function of the RMS value of the AC signals. Thedata acquisition system is connected to the first and second filters toreceive DC signals from the first and second filters.

Another embodiment includes a test apparatus for testing receiverdevices. The test apparatus includes a transmitter source. Thetransmitter source includes an AC supply and a DC supply. Thetransmitter source is connected to a transmitter. The test apparatusfurther includes a fixture configured to receive an optical receiversuch that the optical receiver may receive optical signals from thetransmitter connected to the transmitter source. The optical receivermay include an amplifier with a differential output. The test apparatusalso includes a first matching network able to connect to a high outputof the differential output and a second matching network configured toconnect to a low output of the differential output. Also included in thetest apparatus are a first filter connected to the first matchingnetwork where the first filter and a second filter connected to thesecond matching network. The first and second filters are designed toseparate AC and DC signals. An RMS conversion circuit is connected tothe first and second filters such that the RMS conversion circuitreceives AC signals from the first and second filters. The RMSconversion circuit converts the AC signals to a DC function of the RMSvalue of the AC signals. A data acquisition system is connected to theRMS conversion circuit to receive the function of the RMS value of theAC signals. The data acquisition system is connected to the first andsecond filters to receive DC signals from the first and second filters.

Another embodiment includes a method of testing optical components. Themethod includes receiving an optical signal from an optical source. Theoptical signal including AC and DC components. The optical signal isconverted to an electrical signal. The AC and DC components of theelectrical signal are separated. The AC component of the electricalsignal is converted to a function of the RMS value of the AC componentof the electrical signal. The function of the RMS value is provided to adata acquisition system.

The embodiments described above allow for AC and DC testing to becompleted on optical components using a single inexpensive test board.This can conserve resources as only a single test fixture is used toperform both sets of tests.

These and other advantages and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 illustrates a block diagram overview of a test apparatus fortesting optical components;

FIG. 2 illustrates a schematic drawing showing the general constructionof a test apparatus for optical components; and

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an embodiment of the invention is illustratedas a test apparatus 100 that provides for monitoring of AC and DCsignals produced by a transmitter. The apparatus 100 shown in FIG. 1 maybe used to test transmitter devices. In one example, the transmitterdevice may be a TOSA or a transmitter which includes a laser diode orlight emitting diode (LED). The apparatus shown in FIG. 1 includes adetector device 102. The detector device 102 may be for example adetector that includes a photodiode coupled to a transimpedanceamplifier with a differential output. The detector device 102 will bediscussed in more detail with reference to FIG. 2 described herein. Thedetector device 102 outputs a positive output signal at a positiveoutput signal terminal 104 and a negative output signal at a negativeoutput terminal 106. The positive output signal is fed into a positiveinput port 108 of an RMS detector board 110. Similarly the negativeoutput signal is input to a negative input port 112 of the RMS detectorboard 110.

The RMS detector board 110 includes circuitry for filtering DCcomponents of the output signals from the AC components. The DCcomponents of the signal are output at detector board DC output ports114, 116. The DC output signals may be received by a data acquisitionsystem 118 for use in characterizing the properties of a transmitter.

The AC portions of the signals are converted to a function of the RMSvalue of the AC signals which is then fed to a RMS output port 120. Thefunction of the RMS value of the AC signal may be received by the dataacquisition system 118 and used in characterizing variouscharacteristics of the transmitter.

An alternate embodiment of the invention may be used to test receiverdevices by using a transmitter fixed as a portion of the test apparatus100. In this example, the detector device 102 may be removable such thattesting can be performed on numerous detector devices or receiversproduced by a manufacturer.

Referring now to FIG. 2, a circuit diagram illustrates various featuresof one embodiment. FIG. 2 illustrates a transmitter source 250. Thetransmitter source includes an AC supply 252 and a DC supply 254. The DCsupply 254 may be used to bias a LED or laser diode 256.

The AC supply 252 may be used to modulate the LED or laser diode 256with data or for other reasons. In one embodiment, the AC supply 252 isconfigured to operate at between 1 KHz and 200 MHz. Other embodiments ofthe invention may allow for frequencies up to 2.5 GHz and beyond.

When the test apparatus 200 shown in FIG. 2 is used for testing laserdiodes and LEDs, the transmitter source may be configured to couple to atransmitter such as an LED or laser diode. This configuration mayinclude an appropriate test fixture that allows for quick removal andreplacement of transmitters in the test fixture. The LED or laser diode256 may be optically coupled to a receiver 102. The optical couplingshown in FIG. 2 includes a path through a patch cord 258. This allowsthe optical signal produced by the LED or laser diode 256 to betransmitted to the receiver 102.

The receiver 102 includes a photodiode 260 and a transimpedanceamplifier 262. The photodiode 260 converts optical signals received fromthe patch cord 258 to a small electrical current through the photodiode260. The transimpedance amplifier 262 converts the small current throughthe photodiode 260 into a higher power differential electrical signalthat is output as a differential signal on a positive output 264 and anegative output 268. The differential signal includes a positivedifferential signal 270 and a negative differential signal 272. Thepositive differential signal is fed to a first impedance matchingnetwork 274. The negative differential signal is sent to a secondimpedance matching network 276.

The first and second impedance matching networks 274, 276 may beconfigured to match the line characteristics from the output of thetransimpedance amplifier 262. In one embodiment, the impedance matchingnetworks 274, 276 are fabricated on a printed circuit board thatincludes various paths for receiving different values of components suchas capacitors, resistors and inductors. Thus a printed circuit board canbe customized for a particular test by stuffing the board withappropriately chosen components. Embodiments of the inventioncontemplate the use of several different kinds of matching networks. Forexample and not by way of limitation, a matching network may includespecially designed printed circuit board traces that have a particularcapacitance, inductance and/or resistance. The matching networks mayinclude fixed components such that the matching networks are fixed for aparticular application or use. The matching networks may includeswitched components such that the matching network may be used for aplurality of different applications with minimal reconfiguration. Thematching networks may comprise a variable filter for even furtherflexibility in designing tests apparatus. In one example the matchingnetworks may include a digital signal processor (DSP) that functions asa filter. In some embodiments the first and second impedance matchingnetworks 274, 276 are designed with similar or complementary printedcircuit board layouts and components. This helps to ensure that thepositive differential signal 270 and the negative differential signal272 remain in phase with respect to each other.

The positive differential signal passes through the first impedancematching network 274 to a first filter 278. The first filter 278separates AC and DC signals from the positive differential signal 270.The DC portions of the positive differential signal 270 are fed to a DCoutput 114. The negative differential signal 272 follows a similar paththrough the second impedance matching network 276 to a second filter 280where the DC portion of the negative differential signal 272 is outputat a DC output 116. The first and second filters 278, 280 may be in oneexample bias tees.

The AC output from the first and second filters 278, 280 is fed into anamplifier 282. The amplifier 282 in one embodiment is a high frequencyamplifier with a wide bandwidth, low noise and other desirablecharacteristics. One example of an amplifier that may be used is theAD8129 available from Analog Devices. This particular amplifierfunctions at frequencies up to 250 MHz. The amplifier 282 is thedifferential amplifier that compares the positive AC signal and thenegative AC signal and produces a difference of the two AC signals. Thisdifference of the two AC signal is fed to a true RMS converter 284.

The true RMS converter 284 converts the difference of the AC signals toa function of the RMS value of the difference of the AC signals. In oneembodiment the true RMS converter 284 maybe part number AD8361 fromanalog devices. This particular true RMS converter outputs a signal thatis generally 7.5 times the value of the RMS value of the difference ofthe AC signals. The function of the RMS value of the difference of theAC signals may vary slightly from the 7.5 value depending on theconfiguration of the true RMS converter 284. Alternate functions maybereadily obtained from the data sheet for this device which is availablefrom analog devices on their website. As mentioned previously, thepresent embodiment shown is designed for operation between 1 KHz and 200MHz. However, other embodiments may be designed to function up to 2.5gigahertz and beyond. When embodiments are designed for frequenciesabove 200 MHz, typically the embodiments will be designed as a band passfilter for a range of frequencies so as to obtain the best results fromof the true RMS converter 284. For example, it may be desirable to biasthe true RMS converter 284 such that lower frequencies are less usablewhen the circuit is designed for higher frequencies. When constructingcircuits for use above 200 MHz, an alternate amplifier 282 may be usedthat has a bandwidth suitable for use above 200 MHz.

In constructing the test apparatus 200 it is desirable to construct theapparatus using a printed circuit board layout for certain portions ofthe circuits. It may also be desirable to ensure that traces on theprinted circuit board are matched for positive and negative signalpaths. If the traces are not matched for positive and negative signalpaths, positive and negative signals may vary slightly in their phasefrom each other resulting an erroneous readings from the RMS converter284.

The apparatuses described herein should be calibrated when used as testequipment. However, using the components described herein as testapparatuses has been shown to be very accurate. Thus an automatedcalibration may be used where calibration equipment used in theautomated calibration is calibrated so as to verify the accuracy of theautomated calibration. In one embodiment, a buffer circuit connected tothe output of the true RMS converter 284 may be used to compensate for acircuit that is out of calibration. The buffer circuit may be anamplifier circuit this about unity (i.e. a gain of 1), but adjustableslightly up or down to compensate for any inaccuracies.

The terms high and low frequency, as described herein should beconsidered relative terms rather than applying to a specific standard offrequencies. Thus high-frequency as used herein is used to describecommunications that use relatively high modulation rates as opposed to aspecific range of frequencies as is used in some areas of the electronicand communication arts.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A test apparatus comprising: a transmitter source wherein thetransmitter source comprises: an AC supply; and a DC supply; wherein thetransmitter source is adapted to couple to a transmitter; an opticalreceiver adapted to receive optical signals from a transmitter coupledto the transmitter source, wherein the optical receiver comprises anamplifier with a differential output; a first matching network coupledto a high output of the differential output; a second matching networkcoupled to a low output of the differential output; a first filtercoupled to the first matching network, the first filter being adapted toseparate AC and DC signals; a second filter coupled to the secondmatching network, the second filter being adapted to separate AC and DCsignals; an RMS conversion circuit coupled to the first and secondfilters such that the RMS conversion circuit receives AC signals fromthe first and second filters, wherein the RMS conversion circuitconverts the AC signals to a DC function of the RMS value of the ACsignals; and a data acquisition system coupled to the RMS conversioncircuit to receive the function of the RMS value of the AC signalsfurther wherein the data acquisition system is coupled to the first andsecond filters to receive DC signals from the first and second filters.2. The test apparatus of claim 1, wherein the optical receiver comprisesa photodiode and a transimpedance amplifier.
 3. The test apparatus ofclaim 1, wherein the AC supply is configured to operate at about between1 KHz and 200 MHz.
 4. The test apparatus of claim 1, wherein the firstmatching network and second matching network are matched with similar orcomplementary printed circuit board layouts.
 5. The test apparatus ofclaim 1, wherein the first matching network and second matching networkinclude fixed components.
 6. The test apparatus of claim 1, wherein thefirst matching network and second matching network include a digitalsignal processor.
 7. The test apparatus of claim 1, wherein the firstmatching network and second matching network include switchedcomponents.
 8. The test apparatus of claim 1, wherein the first matchingnetwork and second matching network include traces designed for at leastone of a particular capacitance, inductance and resistance.
 9. The testapparatus of claim 1, wherein the first filter and second filter arebias tees.
 10. The test apparatus of claim 1, wherein the RMS conversioncircuit is included on a printed circuit board, the printed circuitboard adapted to connect to the optical receiver.
 11. The test apparatusof claim 1, wherein the RMS conversion circuit is configured as a bandpass filter designed for a range of frequencies.
 12. The test apparatusof claim 1, further comprising a buffer circuit coupled to the RMSconversion circuit for calibrating the test apparatus.
 13. A testapparatus comprising: a transmitter source wherein the transmittersource comprises: an AC supply; and a DC supply; wherein the transmittersource is coupled to a transmitter; a fixture adapted to receive anoptical receiver such that the optical receiver may receive opticalsignals from the transmitter coupled to the transmitter source, whereinthe optical receiver comprises an amplifier with a differential output;a first matching network adapted to couple to a high output of thedifferential output; a second matching network adapted to couple to alow output of the differential output; a first filter coupled to thefirst matching network, the first filter being adapted to separate ACand DC signals; a second filter coupled to the second matching network,the second filter being adapted to separate AC and DC signals; an RMSconversion circuit coupled to the first and second filters such that theRMS conversion circuit receives AC signals from the first and secondfilters, wherein the RMS conversion circuit converts the AC signals to aDC function of the RMS value of the AC signals; and a data acquisitionsystem coupled to the RMS conversion circuit to receive the function ofthe RMS value of the AC signals further wherein the data acquisitionsystem is coupled to the first and second filters to receive DC signalsfrom the first and second filters.